This invention relates to a method for inhibiting corrosion in boiler feedwater systems and boilers due to dissolved oxygen comprising adding to boiler feedwater chelant-stabilized sodium erythorbate, alone or in combination with scale/deposit inhibitors such as chelants, dispersants, sequestrants, polyelectrolytes, and organic and inorganic phosphates, or conventional boiler corrosion inhibitors such as methoxypropylamine, cyclohexylamine, diethylethanolamine, morpholine, diethyl hydroxylamine, dimethyl amino-2-propanol, 2-amino 2-methylpropanol and carbohydrazide.
More particularly, this invention relates to the use of ethylene diamine tetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) stabilized sodium erythorbate, alone or in combination with conventional boiler scale, deposit and/or corrosion inhibitors, to control corrosion in boiler feedwater systems and boilers.
This invention additionally relates to a method of stabilizing sodium erythorbate against oxidative degradation, and to stabilized sodium erythorbate compositions.
Protection of boiler feedwater systems is becoming an increasingly important aspect of plant operation. The presence of dissolved oxygen in boiler feed water is a primary cause of waterside corrosion. In these energy-conscious times, an increase in the quality of boiler feedwater results in cost savings for the total boiler system.
Historically, the action of dissolved gases such as oxygen and carbon dioxide have been two of the main factors that lead to water feed system and boiler corrosion. In order to understand the role of dissolved gases in corrosion, one must understand the electrochemical nature of corrosion. Under most conditions, there is a tendancy for iron to dissolve in water, and two electrons are released for each iron atom that dissolves. These electrons transfer to hydrogen ions present in the water, and the ions are reduced to elemental gaseous hydrogen. All action ceases at this point if the hydrogen remains on the surface of the metal since a protective coating forms with the passage of electrons. However, any agent which increases the number of hydrogen ions present in the water, or which will cause the removal of the protective film, serves to increase the rate of corrosion.
The presence of oxygen in boiler feedwater causes a two-fold reaction to occur. Some molecules of oxygen combine with displaced hydrogen, thereby exposing the metal to fresh attack. Other oxygen molecules combine with iron ions to form insoluble iron oxide compounds.
The first product of corrosion may be ferric oxide, which is only loosely adherent and aggravates corrosion by blocking off areas to oxygen access. These areas become anionic and iron oxide couples are set up. The iron under the oxide deposit then dissolves, and pitting develops.
With respect to oxygen, the severity of attack will depend on the concentration of dissolved oxygen in the water, water pH and temperature. As water temperature increases, corrosion in feed lines, heaters, boilers, steam and return lines made of iron and steel increases.
The inventors have discovered a new improved method for control of corrosion in boiler feedwater systems and boilers.
A major approach to reducing oxygen in boiler feedwater is mechanical deaeration. Efficient mechanical deaeration can reduce dissolved oxygen to as low as 5-10 ppb in industrial plants and 2-3 ppb in utility operations. However, even with this trace amount of oxygen, some corrosion may occur in boilers. Removal of the last traces of oxygen from boiler feedwater is generally accomplished by the addition of chemicals that react with oxygen and which are hereinafter referred to as oxygen scavengers.
Several oxygen scavengers are known in the art. Widely used oxygen scavengers include, but are not limited to, sodium sulfite, hydrazine, diethylhydroxylamine, carbohydrazide and hydroquinone. U.S. Pat. No. 3,551,349 discloses the use of quinones, particularly hydroquinone, as catalysts for the hydrazine-oxygen reaction. U.S. Pat. No. 4,096,090 discloses the use of hydrazine compounds, a catalytic organometallic complex, and preferably a quinone compound for deoxygenating feedwater. U.S. Pat. No. 3,808,138 discloses the use of cobalt maleic acid hydrazide with hydrazine for oxygen removal. U.S. Pat. No. 3,962,113 discloses the use of organic hydrazine such as monoalkyl hydrazine, dialkyl hydrazine and trialkyl hydrazine as oxygen scavengers.
Disadvantages of hydrazine and related compounds include toxicity and suspected carcinogenic effects. Hydrazine is toxic if inhaled, and is also an irritant to the eyes and skin.
Carbohydrazide, a derivative of hydrazine, decomposes to form hydrazine and carbon dioxide at temperatures above 360.degree. F. U.S. Pat. No. 4,269,717 discloses the use of carbohydrazide as an oxygen scavenger and metal passivator.
U.S. Pat. Nos. 4,278,635 and 4,282,111 disclose the use of hydroquinone, among other dihydroxy, diamino and amino hydroxy benzenes, as oxygen scavengers. U.S. Pat. Nos. 4,279,767 and 4,487,708 disclose the use of hydroquinone and "mu-amines", which are defined as amines which are compatible with hydroquinone. Methoxypropylamine is a preferred mu-amine. U.S. Pat. No. 4,363,734 discloses the use of catalyzed 1,3-dihydroxy acetone as an oxygen scavenger. U.S. Pat. No. 4,419,327 discloses the use of amine or ammonia neutralized erythorbates as oxygen scavengers. Additionally, diethylhydroxylamine (DEHA) has been used as an oxygen scavenger, and U.S. Pat. No. 4,192,844 discloses the use of methoxypropylamine and hydrazine as a corrosion inhibiting composition. European Pat. No. 0054345 discloses the use of amino-phenol compounds or acid addition salts thereof as oxygen scavengers.
UK Patent Application No. 2138796A discloses the use of trivalent phenols, preferably pyrogallol, to improve the activity of hydrazine-trivalent cobalt compositions.